There are a wide variety of structures of a thin film transistor using an oxide semiconductor, for example, a structure of a bottom-gate bottom-contact type (PTL 1) or a bottom-gate top-contact type (PTL 2). In the bottom-gate bottom-contact structure (hereinafter, simply referred to as a bottom-contact structure), a source-drain wiring line (a contact portion with a channel) may be formed through photolithography including a self-alignment process. More specifically, the source-drain wiring line may be formed through a backside exposure process using a gate electrode (a gate wiring line) as a mask. A transparent electroconductive film is used for the source-drain wiring line.
The bottom-contact structure has the following advantages, as compared with the bottom-gate top-contact structure (hereinafter, simply referred to as the top-contact structure). In other words, unlike the top-contact structure, the source-drain wiring line is formed before formation of a semiconductor layer including the channel in the bottom-contact structure. This causes less etching damages to the channel region. Therefore, it is possible to suppress a decrease in reliability and to prevent an increase in variation of the transistor characteristics.